The device of the present invention is specifically intended for use in paying out and recovering electrical cables (and also hoses). However, it is to he appreciated that the present invention is of much wider applicability. Generally, cables are recovered on moving machinery by reeling onto a parallel reel (utilising a layered mechanism to give uniform layering) or onto a radial reel, where, in most cases, the cable lays vertically on a single plane. Other types are "standard" lay, where cable lays horizontally in three then two configuration and random lay where cable lays as it falls.
All the above types are in general use as each has specific advantages either in cost saving, dimensions, or cable tension reduction.
The prime requisite for a cable reel is to payout and recover cables without causing excessive tension in the cable itself. Cable manufacturers have developed cable which can take quite high tensions and are very flexible, but the upper limits of curvature and tension are well defined. Any excursion beyond this limit will lead to premature failure of a very costly item. For short runs of cable and low frequency of use, spring reels were developed and these are quite satisfactory. The demand for reels which could handle long runs of cable, high lift of cable and high duty cycles led to the development of several types Of reel drives, each with claims to satisfactory operation which in many cases were not justified.
The first were "Torque Motors" and hydraulic drives. Both these types are still in use, although hydraulic drives are uncommon. They power the reel in payout and recovery and by the use of sensors which "read" cable tension, try to control the speed to keep cable tension within acceptable limits. The cable tension sensors vary from simply mechanical devices to highly sophisticated micro processor systems with continuous monitoring. Also, control of starting torque and braking torque still seems to be a problem as the inertia of the reel and dries is not fully controlled and cable overtension with subsequent failure is common.
A later development in cable reeling was a friction drive unit which slipped When torque exceeded a certain value. The cable was reeled in under power by the drive unit and payed out by the cable driving the reel. One of these units used a spring loaded steel disc bearing on one side of a bronze worm in the drive unit. Lubrication was by the oil normally in the gear unit and the whole unit was enclosed. This unit depends on the slipping faces being lubricated at all times to prevent wear.
However, to prevent hydro-dynamic lubrication of the rubbing faces, where friction would be too low to be practical, fairly high unit pressures are used. While running, the drive gives a reasonably constant torque during payout and recovery, but if the cycle is interrupted for a short time, the oil film is partly extruded from between the slipping faces, the slipping faces contact each other and friction increases. On restarting, the payout cable tension to overcome this friction rises up to five times the normal running tension and this is rarely acceptable.
Further, if air cooling is used for the slipping faces, the units become larger and more expensive.
A later development of the friction drive was an air-cooled disc with circular asbestos friction pads uniformly loaded onto the disc with stacks of Belville spring washers. Each spring stack had a torque range and the torque could be adjusted within this range by a gear and threaded pressure plate. Being dry, this unit did not have the fault of high starting torque inherent in the oil lubricated unit. The unit was ideal within its torque and power range and, although fairly complicated and expensive, was reliable and had an excellent working life. The banning of the use of asbestos meant that the pad friction lining material had to be changed. The performance of the unit remained unchanged but the life of the friction pads and discs was considerably reduced and any competitive edge was lost. Research is continuing for a suitable friction material. The present invention seeks to overcome or at least alleviate some of the aforementioned problems of the prior art.